Cavern

A cavern is a large subterranean cavity. Almost all caverns with two
or more entrances technically qualify as natural arches. They are included
in the taxonomy as a morphologic type even though it is usually easy
to determine how any given cavern formed. The attribute that distinguishes
a cavern from other natural arches is a morphologic attribute. Light
entering the opening, including diffused and reflected light, does not
reach all parts of it, i.e., an observer can be positioned in the opening
such that they are in total darkness during broad daylight. Using the
standard attribute list, a cavern is a natural arch that has the morphologic
attribute of being cavernous.

Caverns can form in many different ways. As stated above, the geologic
processes that contributed to the formation of any given cavern are
usually obvious. However, about the only caverns that have much interest
as natural arches are waterfall natural
bridges with cavernous subterranean passageways. Upon occasion,
some of these have been labeled by the public as natural tunnels or
even natural bridges. Even though it is clear that these features formed
exactly like non-cavernous waterfall natural bridges, they are included
in the taxonomy as caverns. Indeed, any natural arch that is cavernous
is considered a cavern regardless of how it formed.